1. Field of the Invention
This invention relates to a process for reducing the sulfur oxide content of a waste gas stream by using absorbents which can be reactivated for further absorption of sulfur oxides by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst. More particularly, this invention relates to a method for reducing sulfur oxide emissions from the regenerator of a fluidized catalytic cracking unit.
2. Setting of the Invention
A major industrial problem invoves the development of efficient methods for reducing the concentration of air pollutants, such as sulfur oxides, in the waste gases which result from the processing and combustion of sulfur containing fuels. The discharge of these waste gases into the atmosphere is environmentally undesirable at the sulfur oxide concentrations which are frequently encountered. For example, such waste gases result from combustion of sulfur containing fossil fuels for the generation of heat and power, the regeneration of catalysts employed in the refining of hydrocarbon feedstocks which contain organic sulfur compounds, and the operation of Claus-type sulfur recovery units.
Two basic approaches have been suggested for the removal of sulfur oxides from a waste gas. One approach involves scrubbing the waste gas with an alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to produce a nonvolatile waste product. This approach requires a large supply of the alkaline scrubbing material, and the resulting reaction waste products may create a solid waste disposal problem. The second basic approach uses sulfur oxide absorbents which can be regenerated either thermally or chemically. The process of this invention is representative of this second approach.
One area of interest for reduction of sulfur oxides from waste gas which uses the second basic approach is cyclic, fluidized catalytic cracking of petroleum. This process involves the cracking of a petroleum feedstock in a reaction zone through contact with fluidized solid particles of a cracking catalyst. The cracking catalyst becomes substantially deactivated by nonvolatile coke deposits and is separated from the reaction zone effluent and stripped of volatile deposits in a stripping zone. The stripped cracking catalyst particles are separated from the stripping zone effluent, regenerated in a regeneration zone by combustion of the coke with an oxygen containing gas, and thereafter the regenerated catalyst particles are returned to the reaction zone. If sulfur-containing feedstocks are used in this process, the cracking catalyst will become deactivated through the formation of sulfur-containing deposits of coke. In conventional processes, the combustion of this sulfur-containing coke results in the release of substantial amounts of sulfur oxides to the atmosphere.
Various methods have been used to reduce sulfur oxide emissions by utilizing different types of reactants with the cracking catalyst. A commonly utilized reactant is an alumina matrix combined with rare earth metals. Many other metals have been tried; however, to the best of our knowledge yttria (an oxide of yttrium) has never been used as a reactant for this disclosed purpose.
U.S. Pat. No. 3,835,031, to R. J. Bertolacini et al. discloses a method for the reduction of the sulfur oxide emissions through the use of a cracking catalyst comprising a zeolite in a silica-alumina matrix which has from about 0.25 to about 5.0 weight percent of a Group IIA metal or mixture of Group IIA metals distributed over the surface of the matrix and present as an oxide or oxides. The metal oxide or oxides react with sulfur oxides in the regeneration zone to form nonvolatile inorganic sulfur compounds. These nonvolatile inorganic sulfur compounds are then converted to the metal oxide or oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and stripping zones of the process unit. The resulting hydrogen sulfide is disposed of in equipment conventionally associated with a fluid catalytic cracking unit. Similarly, Belgian Pat. No. 849,637 also is directed to a process wherein a Group IIA metal or metals is circulated through a cyclic fluidized catalytic cracking process in order to reduce the sulfur oxide emissions resulting from regeneration of deactivated catalyst. These patents do not suggest the use of yttrium or combining yttrium with a rare earth metal for use as a reactant.
Belgian Pat. No. 849,636 and its counterpart, U.S. Pat. application Ser. No. 748,556, disclose a process similar to that set forth in U.S. Pat. No. 3,835,031, which involves the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized, catalytic cracking unit through the use of a zeolite-type cracking catalyst in combination with a regenerable metallic reactant. The reactant absorbs sulfur oxides in the regeneration zone and releases the absorbed sulfur oxides as hydrogen sulfide in the reaction and stripping zones of the process unit. It is taught therein that a suitable metallic reactant comprises one or more members selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rare earth metals, and lead, all in free or combined form. In addition, it is disclosed that the metallic reactant may be supported by an amorphous cracking catalyst or a solid which is substantially inert to the cracking reaction. Silica, alumina and mixtures of silica and alumina are mentioned as suitable supports. There is no specific teaching, however, of the desirability of combining any particular rare earth metals with inorganic oxides selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium and calcium. There is no mention of the use of yttrium as a suitable reactant. Further, the disclosure contains no suggestion that such a combination would afford a synergistically enhanced reduction of regenerator sulfur oxide emissions.
Belgian Pat. No. 849,635 and its counterpart, U.S. patent application Ser. No. 748,555 are also directed to a similar process to U.S. Pat. No. 3,835,031 and Belgian Pat. No. 849,636, and teaches that an improved reduction of regeneration zone sulfur oxide emissions can be achieved by combining a sulfur oxide absorbent with a metallic promoter, including platinum and palladium. The sulfur oxide absorbent comprises at least one free or combined element which is selected from the group consisting of sodium, magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. Although the metallic promoter enhances the ability of the absorbent to absorb sulfur oxides in the regeneration zone of a cyclic, fluidized, catalytic cracking unit, the more active promoters, such as platinum and palladium, also promote the formation of nitrogen oxides and the combustion of carbon monoxide in the regeneration zone. Since the discharge of nitrogen oxides into the atmosphere is environmentally undesirable, the use of these promoters is unattractive. The ability of these promoters to enhance the combustion of carbon monoxide in the regenerator is also undesirable in those situations wherein the regenerator vessel and associated equipment, such as cyclones and flue gas lines, are constructed of metals, such as carbon steel, which may not be able to tolerate the increased regeneration temperatures which can result from enhanced carbon monoxide combustion. It is not disclosed therein to use yttrium as a reactant to reduce sulfur oxide emissions.
U.S. Pat. No. 4,146,463 to H. D. Radford et al. discloses a process wherein a waste gas which includes sulfur oxides and/or carbon monoxide is conveyed to the regeneration zone of a cyclic, fluidized, catalytic cracking unit wherein it is contacted with a metal oxide which reacts with the sulfur oxides to form nonvolatile inorganic sulfur compounds. This patent teaches that suitable metal oxides include those selected from the group consisting of the oxides of sodium, the Group IIA metals, scandium, titanium, chromium, iron, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals. In addition, the patent teaches that the metal oxide may be incorporated into or deposited onto a suitable support such as silica, alumina and mixtures of silica and alumina. The teaching of this patent fails to suggest the use of yttrium with one or more inorganic oxides selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium and calcium.
U.S. Pat. No. 4,071,436 to W. A. Blanton et al. teaches that alumina and/or magnesia can be used to absorb sulfur oxides from a gas and the absorbed sulfur oxides can be removed by treatment with a hydrocarbon. It is further disclosed therein that sulfur oxide emissions from the regenerator of a cyclic, fluidized, catalytic cracking unit can be reduced by combining alumina and/or magnesia with the hydrocarbon cracking catalyst. Similarly, U.S. Pat. Nos. 4,115,249 and 4,115,251 teach the utility of alumina or aluminum to absorb sulfur oxides in the regenerator of a cyclic, fluidized, catalytic cracking unit. The disclosures of these patents do not, however, mention yttrium or rare earth metals in combination with alumina and/or magnesia to give improved results.
U.S. Pat. No. 4,001,375 to J. M. Longo discloses a process for removal of sulfur oxides from a gas which involves absorbing the sulfur oxides with cerium oxide followed by regeneration of the spent cerium oxide by reaction with hydrogen gas. It is further disclosed that the cerium oxide may be supported on an inert support such as alumina, silica and magnesia. The patent does not, however, suggest that the use of another type of reactant, such as yttrium, or that the spent cerium oxide could be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst. In addition, the patent fails to suggest that the reactant can be combined with alumina and/or magnesia to effect an absorption of sulfur oxides.
U.S. patent application Ser. No. 29,264 to Bertolacini et al. discloses a composition of material and a process for removal of sulfur oxides from a gas. Specifically, sulfur oxides are removed from a gas by an absorbent comprising an inorganic oxide in association with at least one free or combined rare earth metal. The absorbed sulfur oxides are recovered as a sulfur-containing gas by contacting the spent absorbent with a hydrocarbon in the presence of a hydrocarbon cracking catalyst. There is no disclosure in Ser. No. 29,264 for the use of yttrium as an absorbent in a fluidized catalytic cracking process.
U.S. Pat. No. 4,311,581 discloses an article entitled "Selection of Metal Oxides for Removing SO.sub.2 from Flue Gas" by Lowell et al. in Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971, is addressed to a theoretical evaluation of the possible use of various metal oxides to absorb sulfur dioxide from a flue gas. The authors evaluate 47 metal oxides from which they select a group of 16 potentially useful single oxide absorbents, which includes the oxides of aluminum, cerium and titanium. Yttrium was not included in the list. The absorbents are taught to be regenerated thermally and the paper does not consider the possibility of regeneration under reducing conditions. Consequently, there is no suggestion that any of the metal oxides could be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.
U.S. Pat. No. 3,899,444 to R. E. Stephens is directed to the preparation of a catalyst support which consists of an inert substrate or core which is coated with an alumina containing from about 1 to about 45 weight percent, based on the alumina, of a rare earth metal oxide which is uniformly distributed throughout the alumina coating. It is disclosed that the inert substrate may include such refractory materials as zirconia, zinc oxide, alumina-magnesia, calcium aluminate, synthetic and natural zeolites among many others. Yttrium is not listed in the list of rare earth metals contemplated. See Col. 5, lines 9-14. Similarly, U.S. Pat. No. 4,062,810 to W. Vogt et al. discloses compositions comprising cerium oxide on an aluminum oxide support. Neither of these patents teach the use of yttrium.
U.S. Pat. No. 3,823,092 to E. M. Gladrow describes the treatment of a zeolite-type hydrocarbon cracking catalyst with a dilute solution containing cerium cations or a mixture of rare earth cations having a substantial amount of cerium in order to improve the regeneration rate of the catalyst. The resulting catalyst contains between about 0.5 and 4.0 percent of cerium oxide and it is further disclosed that the catalyst matrix may contain from 5 to 30% alumina. Similarly, U.S. Pat. No. 3,930,987 to H. S. Grand describes a hydrocarbon cracking catalyst comprising a composite of a crystalline aluminosilicate carrying rare earth metal cations dispersed in an inorganic oxide matrix wherein at least 50 weight percent of the inorganic is silica and/or alumina, and the rare earth metal content of the matrix is from 1 to 6 percent expressed as RE.sub.2 O.sub.3. At Col. 7, lines 60-64, Grand includes yttrium as a rare earth metal. And at Col. 8, lines 5-21, yttrium is included in rare earth chloride solutions as a very minor substituent, i.e. 0.4% by weight of the diodymium chloride. Nowhere is it disclosed to use yttrium with specific metal oxides as a sulfur oxide absorbent. Also, U.S. Pat. No. 4,137,151 to S. M. Csicsery discloses a composition comprising lanthanum or a lanthanum compound in association with a porous inorganic oxide which may be the matrix of a zeolite-type cracking catalyst. These patents contain no mention of sulfur oxides and fail to suggest that the combination of yttrium or specific rare earth metals with specific metal oxides, such as alumina, could afford an improved sulfur oxide absorbent which can be regenerated by contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.